Update from Gnulib

This incorporates:
2019-05-24 flexmember: update comments
2019-05-18 pthread_sigmask: fix --enable-threads=windows compilation
2019-05-14 close-stream, closein, closeout: simplify
2019-05-09 verify: remove verify_true
2019-05-09 verify: support C2X and C++17 static_assert
* build-aux/config.guess, build-aux/config.sub:
* doc/misc/texinfo.tex, lib/flexmember.h, lib/verify.h:
* m4/flexmember.m4, m4/pthread_sigmask.m4:
Copy from Gnulib
* m4/gnulib-comp.m4: Regenerate.

2019-05-26  Paul Eggert  <eggert@cs.ucla.edu>

Update author/maintainer info
This mostly updates email addresses and fixes spellings of
author and maintainer names.
1 job for master in 51 minutes and 17 seconds (queued for 3 seconds)
Status Job ID Name Coverage
failed #1940


Name Stage Failure
test-all Test
Makefile:318: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:293: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
make: *** [check-expensive] Error 2
Makefile:957: recipe for target 'check-expensive' failed
ERROR: Job failed: exit code 1